The most Avoided Fact about HVAC filters is bleed-trough. This is because it is hardly ever mentioned and most filter folks would rather focus on filter by-pass, which is important to discuss but not nearly as detrimental and is more easily remedied than bleed-through. So why is by-pass the shiny object most sales people want you to focus on? Simple - it is more easily shown and doesn’t require an in-depth look at the filters, and chances are they are selling you a filter that may not have by-pass but suffers significant bleed-through.
It is well known that a gap as small as 1/8” around an HVAC filter allows as much dirty air to by-pass around the filter as a 3” hole right in the middle, decreasing filter efficiency by 20%-30%. With the purpose of an air filter to clean the air by-pass presents a serious problem that must be addressed by making sure filter racks are kept in good condition and the filters are properly sized for your air handler. Just as insidious and much less discussed however is filter bleed-through. Unlike by-pass which is easy to see visually if filters are fitting properly into a filter rack, bleed-through typically needs the aid of a particle counter as the naked eye can only see particles larger than 100 microns and filters are rated at particles smaller than 10 microns.
Bleed-Trough: Non-partisan ASHRAE states on page 3 of the 52.2–2012 standard that filters using a synthetic filtration media exhibit higher a MERV during testing than what they will achieve during actual use. These filters enhance their particle capture by adding tackifiers, electrostatic charges, and mechanical charges to coarse fibers. This creates a filter that tests well in a lab, but on-site particle counts will show a degradation in efficiency. We have tested some MERV-13 filters that fell to MERV-8 in only a couple months - most MERV-13 filters should last a year or more. Some of the low-efficiency MERV-8s we’ve seen to lose half their filtration ability. All of these filters fit well in a frame and some even have extra gaskets or a “self-sealing” edge that eliminates by-pass. Because bleed-through is not easy to see upon visual inspection it often goes uncorrected. Most reputable air filter suppliers should be able to test your filters for you with the aid of a laser particle counter, or provide you with direction on who you could contact to get filters tested for bleed-through - on-site data collection is highly recommended so you can see the procedure for yourself.
By-Pass: Filters that exhibit bypass are very easy to detect, cheap pleated and panel style filters are the biggest offenders of filter by-pass as the cardboard frame can degrade while the filter is in service, however there are some out there with better frames to compensate for this notorious weak spot. If quality products are used, and filter frames/racks are kept gasketed then there should be no issue with by-pass. This is one of the most easily correctable issues that an HVAC filter may have.
Real Filter Efficiency The end result of having filters that suffer from Bleed-Trough or Bypass is a loss in efficiency; however, particle capture efficiency values - as reported by MERV per ASHRAE 52.2–2012 standard - can be a bit misleading to start with. The overall efficiency of all particles in the air MERV-8 catches 0.51% of all particles, MERV-11 catches 20.77% of all particles, and MERV-13 catches 50.60% of all particles per the 52.2–2012 MERV efficiency table. So when you look at eliminating filter by-pass and bleed-through the largest impact will be on the higher efficiency filters, with MERV-8 you drop from an already low 0.51% total efficiency to 0.36%; BUT at MERV-13 you would drop from catching over half of all particles to catching slightly over one-third of all particles.
Some good news: ISO16890 is a new air filter test method that will help correct some of these issues, and we here at Filtration Systems are ready.